Organometallic complexes as phosphorescent emitters in organic LEDs

ABSTRACT

Organic light emitting devices are described wherein the emissive layer comprises a host material containing an emissive molecule, which molecule is adapted to luminesce when a voltage is applied across the heterostructure, and the emissive molecule is selected from the group of phosphorescent organometallic complexes, including cyclometallated platinum, iridium and osmium complexes. The organic light emitting devices optionally contain an exciton blocking layer. Furthermore, improved electroluminescent efficiency in organic light emitting devices is obtained with an emitter layer comprising organometallic complexes of transition metals of formula L 2 MX, wherein L and X are distinct bidentate ligands. Compounds of this formula can be synthesized more facilely than in previous approaches and synthetic options allow insertion of fluorescent molecules into a phosphorescent complex, ligands to fine tune the color of emission, and ligands to trap carriers.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/233,605, filed Sep. 22, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/870,788, filed Jun. 16, 2004, now U.S. Pat. No.7,001,536, which is a division of application Ser. No. 10/171,235, filedJun. 13, 2002, now U.S. Pat. No. 6,902,830, and a continuation ofapplication Ser. No. 09/883,734, filed Jun. 18, 2001, now U.S. Pat. No.6,830,828, which is a continuation-in-part of application Ser. No.09/452,346, filed Dec. 1, 1999, now abandoned, and acontinuation-in-part of application Ser. No. 09/311,126, filed May 13,1999, now abandoned, and a continuation-in-part of application Ser. No.09/274,609, filed Mar. 23, 1999, now abandoned, the contents of whichare incorporated by reference in their entirety.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.F33615-94-1-1414 awarded by DARPA. The government has certain rights inthis invention.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, The University ofSouthern California, The University of Michigan, and Universal DisplayCorporation. The agreement was in effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of the agreement.

FIELD OF INVENTION

The present invention is directed to organic light emitting devices(OLEDs) comprised of emissive layers that contain an organometallicphosphorescent compound.

BACKGROUND OF THE INVENTION

Organic light emitting devices (OLEDs) are comprised of several organiclayers in which one of the layers is comprised of an organic materialthat can be made to electroluminesce by applying a voltage across thedevice, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. CertainOLEDs have been shown to have sufficient brightness, range of color andoperating lifetimes for use as a practical alternative technology toLCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrowsand M. E. Thompson, Laser Focus World, February 1995). Since many of thethin organic films used in such devices are transparent in the visiblespectral region, they allow for the realization of a completely new typeof display pixel in which red (R), green (G), and blue (B) emittingOLEDs are placed in a vertically stacked geometry to provide a simplefabrication process, a small R-G-B pixel size, and a large fill factor,International Patent Application No. PCT/US95/15790.

A transparent OLED (TOLED), which represents a significant step towardrealizing high resolution, independently addressable stacked R-G-Bpixels, was reported in International Patent Application No.PCT/US97/02681 in which the TOLED had greater than 71% transparency whenturned off and emitted light from both top and bottom device surfaceswith high efficiency (approaching 1% quantum efficiency) when the devicewas turned on. The TOLED used transparent indium tin oxide (ITO) as thehole-injecting electrode and a Mg—Ag-ITO electrode layer forelectron-injection. A device was disclosed in which the ITO side of theMg—Ag-ITO electrode layer was used as a hole-injecting contact for asecond, different color-emitting OLED stacked on top of the TOLED. Eachlayer in the stacked OLED (SOLED) was independently addressable andemitted its own characteristic color. This colored emission could betransmitted through the adjacently stacked, transparent, independentlyaddressable, organic layer or layers, the transparent contacts and theglass substrate, thus allowing the device to emit any color that couldbe produced by varying the relative output of the red and bluecolor-emitting layers.

The PCT/US95/15790 application disclosed an integrated SOLED for whichboth intensity and color could be independently varied and controlledwith external power supplies in a color tunable display device. ThePCT/US95/15790 application, thus, illustrates a principle for achievingintegrated, full color pixels that provide high image resolution, whichis made possible by the compact pixel size. Furthermore, relatively lowcost fabrication techniques, as compared with prior art methods, may beutilized for making such devices.

Because light is generated in organic materials from the decay ofmolecular excited states or excitons, understanding their properties andinteractions is crucial to the design of efficient light emittingdevices currently of significant interest due to their potential uses indisplays, lasers, and other illumination applications. For example, ifthe symmetry of an exciton is different from that of the ground state,then the radiative relaxation of the exciton is disallowed andluminescence will be slow and inefficient. Because the ground state isusually anti-symmetric under exchange of spins of electrons comprisingthe exciton, the decay of a symmetric exciton breaks symmetry. Suchexcitons are known as triplets, the term reflecting the degeneracy ofthe state. For every three triplet excitons that are formed byelectrical excitation in an OLED, only one symmetric state (or singlet)exciton is created. (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S.R. Forrest, Very high-efficiency green organic light-emitting devicesbased on electrophosphorescence, Applied Physics Letters, 1999, 75,4-6.) Luminescence from a symmetry-disallowed process is known asphosphorescence. Characteristically, phosphorescence may persist for upto several seconds after excitation due to the low probability of thetransition. In contrast, fluorescence originates in the rapid decay of asinglet exciton. Since this process occurs between states of likesymmetry, it may be very efficient.

Many organic materials exhibit fluorescence from singlet excitons.However, only a very few have been identified which are also capable ofefficient room temperature phosphorescence from triplets. Thus, in mostfluorescent dyes, the energy contained in the triplet states is wasted.However, if the triplet excited state is perturbed, for example, throughspin-orbit coupling (typically introduced by the presence of a heavymetal atom), then efficient phosphorescence is more likely. In thiscase, the triplet exciton assumes some singlet character and it has ahigher probability of radiative decay to the ground state. Indeed,phosphorescent dyes with these properties have demonstrated highefficiency electroluminescence.

Only a few organic materials have been identified which show efficientroom temperature phosphorescence from triplets. In contrast, manyfluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang, “Recentdevelopments in molecular organic electroluminescent materials,”Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, LambdachromeLaser Dyes (Lambda Physik, Gottingen, 1997)) and fluorescentefficiencies in solution approaching 100% are not uncommon. (C. H. Chen,1997, op. cit.) Fluorescence is also not affected by triplet-tripletannihilation, which degrades phosphorescent emission at high excitationdensities. (M. A. Baldo, et al., “High efficiency phosphorescentemission from organic electroluminescent devices,” Nature, 1998, 395,151-154; M. A. Baldo, M. E. Thompson, and S. R. Forrest, “An analyticmodel of triplet-triplet annihilation in electrophosphorescent devices,”1999). Consequently, fluorescent materials are suited to manyelectroluminescent applications, particularly passive matrix displays.

To understand the different embodiments of this invention, it is usefulto discuss the underlying mechanistic theory of energy transfer. Thereare two mechanisms commonly discussed for the transfer of energy to anacceptor molecule. In the first mechanism of Dexter transport (D. L.Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys.,1953, 21, 836-850), the exciton may hop directly from one molecule tothe next. This is a short-range process dependent on the overlap ofmolecular orbitals of neighboring molecules. It also preserves thesymmetry of the donor and acceptor pair (E. Wigner and E. W. Wittmer,Uber die Struktur der zweiatomigen Molekelspektren nach derQuantenmechanik, Zeitschrift fur Physik, 1928, 51, 859-886; M.Klessinger and J. Michl, Excited states and photochemistry of organicmolecules (VCH Publishers, New York, 1995)). Thus, the energy transferof Eq. (1) is not possible via Dexter mechanism. In the second mechanismof Förster transfer (T. Förster, Zwischenmolekulare Energiewanderung andFluoreszenz, Annalen der Physik, 1948, 2, 55-75; T. Förster, Fluoreszenzorganischer Verbindugen (Vandenhoek and Ruprecht, Gottinghen, 1951), theenergy transfer of Eq. (1) is possible. In Förster transfer, similar toa transmitter and an antenna, dipoles on the donor and acceptormolecules couple and energy may be transferred. Dipoles are generatedfrom allowed transitions in both donor and acceptor molecules. Thistypically restricts the Förster mechanism to transfers between singletstates.

Nevertheless, as long as the phosphor can emit light due to someperturbation of the state such as due to spin-orbit coupling introducedby a heavy metal atom, it may participate as the donor in Förstertransfer. The efficiency of the process is determined by the luminescentefficiency of the phosphor (F Wilkinson, in Advances in Photochemistry(eds. W. A. Noyes, G. Hammond, and J. N. Pitts), pp. 241-268, John Wiley& Sons, New York, 1964), i.e., if a radiative transition is moreprobable than a non-radiative decay, then energy transfer will beefficient. Such triplet-singlet transfers were predicted by Förster (T.Förster, “Transfer mechanisms of electronic excitation,” Discussions ofthe Faraday Society, 1959, 27, 7-17) and confirmed by Ermolaev andSveshnikova (V. L. Ermolaev and E. B. Sveshnikova, “Inductive-resonancetransfer of energy from aromatic molecules in the triplet state,”Doklady Akademii Nauk SSSR, 1963, 149, 1295-1298), who detected theenergy transfer using a range of phosphorescent donors and fluorescentacceptors in rigid media at 77 K or 90 K. Large transfer distances areobserved; for example, with triphenylamine as the donor and chrysoidineas the acceptor, the interaction range is 52 Å.

The remaining condition for Förster transfer is that the absorptionspectrum should overlap the emission spectrum of the donor assuming theenergy levels between the excited and ground state molecular pair are inresonance. In one example of this application, we use the green phosphorfac tris(2-phenylpyridine) iridium (Ir(ppy)₃; M. A. Baldo, et al., Appl.Phys. Lett., 1999, 75, 4-6) and the red fluorescent dye[2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-ylidene]propane-dinitrile](“DCM2”; C. W. Tang, S. A. VanSlyke, and C. H. Chen,“Electroluminescence of doped organic films,” J. Appl. Phys., 1989, 65,3610-3616). DCM2 absorbs in the green, and, depending on the localpolarization field (V. Bulovic, et al., “Bright, saturated,red-to-yellow organic light-emitting devices based onpolarization-induced spectral shifts,” Chem. Phys. Lett., 1998, 287,455-460), it emits at wavelengths between λ=570 nm and λ=650 nm.

It is possible to implement Förster energy transfer from a triplet stateby doping a fluorescent guest into a phosphorescent host material.Unfortunately, such systems are affected by competitive energy transfermechanisms that degrade the overall efficiency. In particular, the closeproximity of the host and guest increase the likelihood of Dextertransfer between the host to the guest triplets. Once excitons reach theguest triplet state, they are effectively lost since these fluorescentdyes typically exhibit extremely inefficient phosphorescence.

To maximize the transfer of host triplets to fluorescent dye singlets,it is desirable to maximize Dexter transfer into the triplet state ofthe phosphor while also minimizing transfer into the triplet state ofthe fluorescent dye. Since the Dexter mechanism transfers energy betweenneighboring molecules, reducing the concentration of the fluorescent dyedecreases the probability of triplet-triplet transfer to the dye. On theother hand, long range Förster transfer to the singlet state isunaffected. In contrast, transfer into the triplet state of the phosphoris necessary to harness host triplets, and may be improved by increasingthe concentration of the phosphor.

Devices whose structure is based upon the use of layers of organicoptoelectronic materials generally rely on a common mechanism leading tooptical emission. Typically, this mechanism is based upon the radiativerecombination of a trapped charge. Specifically, OLEDs are comprised ofat least two thin organic layers separating the anode and cathode of thedevice. The material of one of these layers is specifically chosen basedon the material's ability to transport holes, a “hole transportinglayer” (HTL), and the material of the other layer is specificallyselected according to its ability to transport electrons, an “electrontransporting layer” (ETL). With such a construction, the device can beviewed as a diode with a forward bias when the potential applied to theanode is higher than the potential applied to the cathode. Under thesebias conditions, the anode injects holes (positive charge carriers) intothe hole transporting layer, while the cathode injects electrons intothe electron transporting layer. The portion of the luminescent mediumadjacent to the anode thus forms a hole injecting and transporting zonewhile the portion of the luminescent medium adjacent to the cathodeforms an electron injecting and transporting zone. The injected holesand electrons each migrate toward the oppositely charged electrode. Whenan electron and hole localize on the same molecule, a Frenkel exciton isformed. Recombination of this short-lived state may be visualized as anelectron dropping from its conduction potential to a valence band, withrelaxation occurring, under certain conditions, preferentially via aphotoemissive mechanism. Under this view of the mechanism of operationof typical thin-layer organic devices, the electroluminescent layercomprises a luminescence zone receiving mobile charge carriers(electrons and holes) from each electrode.

As noted above, light emission from OLEDs is typically via fluorescenceor phosphorescence. There are issues with the use of phosphorescence. Ithas been noted that phosphorescent efficiency decreases rapidly at highcurrent densities. It may be that long phosphorescent lifetimes causesaturation of emissive sites, and triplet-triplet annihilation mayproduce efficiency losses. Another difference between fluorescence andphosphorescence is that energy transfer of triplets from a conductivehost to a luminescent guest molecule is typically slower than that ofsinglets; the long range dipole-dipole coupling (Förster transfer) whichdominates energy transfer of singlets is (theoretically) forbidden fortriplets by the principle of spin symmetry conservation. Thus, fortriplets, energy transfer typically occurs by diffusion of excitons toneighboring molecules (Dexter transfer); significant overlap of donorand acceptor excitonic wavefunctions is critical to energy transfer.Another issue is that triplet diffusion lengths are typically long(e.g., >1400 Å) compared with typical singlet diffusion lengths of about200 Å. Thus, if phosphorescent devices are to achieve their potential,device structures need to be optimized for triplet properties. In thisinvention, we exploit the property of long triplet diffusion lengths toimprove external quantum efficiency.

Successful utilization of phosphorescence holds enormous promise fororganic electroluminescent devices. For example, an advantage ofphosphorescence is that all excitons (formed by the recombination ofholes and electrons in an EL), which are (in part) triplet-based inphosphorescent devices, may participate in energy transfer andluminescence in certain electroluminescent materials. In contrast, onlya small percentage of excitons in fluorescent devices, which aresinglet-based, result in fluorescent luminescence.

An alternative is to use phosphorescence processes to improve theefficiency of fluorescence processes. Fluorescence is in principle 75%less efficient due to the three times higher number of symmetric excitedstates.

Because one typically has at least one electron transporting layer andat least one hole transporting layer, one has layers of differentmaterials, forming a heterostructure. The materials that produce theelectroluminescent emission are frequently the same materials thatfunction either as the electron transporting layer or as the holetransporting layer. Such devices in which the electron transportinglayer or the hole transporting layer also functions as the emissivelayer are referred to as having a single heterostructure. Alternatively,the electroluminescent material may be present in a separate emissivelayer between the hole transporting layer and the electron transportinglayer in what is referred to as a double heterostructure. The separateemissive layer may contain the emissive molecule doped into a host orthe emissive layer may consist essentially of the emissive molecule.

That is, in addition to emissive materials that are present as thepredominant component in the charge carrier layer, that is, either inthe hole transporting layer or in the electron transporting layer, andthat function both as the charge carrier material as well as theemissive material, the emissive material may be present in relativelylow concentrations as a dopant in the charge carrier layer. Whenever adopant is present, the predominant material in the charge carrier layermay be referred to as a host compound or as a receiving compound.Materials that are present as host and dopant are selected so as to havea high level of energy transfer from the host to the dopant material. Inaddition, these materials need to be capable of producing acceptableelectrical properties for the OLED. Furthermore, such host and dopantmaterials are preferably capable of being incorporated into the OLEDusing starting materials that can be readily incorporated into the OLEDby using convenient fabrication techniques, in particular, by usingvacuum-deposition techniques.

The exciton blocking layer used in the devices of the present invention(and previously disclosed in U.S. application Ser. No. 09/153,144, nowU.S. Pat. No. 6,097,147) substantially blocks the diffusion of excitons,thus substantially keeping the excitons within the emission layer toenhance device efficiency. The material of blocking layer of the presentinvention is characterized by an energy difference (“band gap”) betweenits lowest unoccupied molecular orbital (LUMO) and its highest occupiedmolecular orbital (HOMO). In accordance with the present invention, thisband gap substantially prevents the diffusion of excitons through theblocking layer, yet has only a minimal effect on the turn-on voltage ofa completed electroluminescent device. The band gap is thus preferablygreater than the energy level of excitons produced in an emission layer,such that such excitons are not able to exist in the blocking layer.Specifically, the band gap of the blocking layer is at least as great asthe difference in energy between the triplet state and the ground stateof the host.

It is desirable for OLEDs to be fabricated using materials that provideelectroluminescent emission in a relatively narrow band centered nearselected spectral regions, which correspond to one of the three primarycolors, red, green and blue so that they may be used as a colored layerin an OLED or SOLED. It is also desirable that such compounds be capableof being readily deposited as a thin layer using vacuum depositiontechniques so that they may be readily incorporated into an OLED that isprepared entirely from vacuum-deposited organic materials.

U.S. patent application Ser. No. 08/774,087, filed Dec. 23, 1996, nowU.S. Pat. No. 6,048,630, is directed to OLEDs containing emittingcompounds that produce a saturated red emission.

SUMMARY OF THE INVENTION

The present invention is directed to organic light emitting deviceswherein the emissive layer comprises an emissive molecule, optionallywith a host material (wherein the emissive molecule is present as adopant in said host material), which molecule is adapted to luminescewhen a voltage is applied across the heterostructure, wherein theemissive molecule is selected from the group of phosphorescentorganometallic complexes. The emissive molecule may be further selectedfrom the group of phosphorescent organometallic platinum, iridium, orosmium complexes and may be still further selected from the group ofphosphorescent cyclometallated platinum, iridium, or osmium complexes. Aspecific example of the emissive molecule is fac tris(2-phenylpyridine)iridium, denoted (Ir(ppy)₃) of formula

[In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.]

The general arrangement of the layers is hole transporting layer,emissive layer, and electron transporting layer. For a hole conductingemissive layer, one may have an exciton blocking layer between theemissive layer and the electron transporting layer. For an electronconducting emissive layer, one may have an exciton blocking layerbetween the emissive layer and the hole transporting layer. The emissivelayer may be equal to the hole transporting layer (in which case theexciton blocking layer is near or at the anode) or to the electrontransporting layer (in which case the exciton blocking layer is near orat the cathode).

The emissive layer may be formed with a host material in which theemissive molecule resides as a guest or the emissive layer may be formedof the emissive molecule itself. In the former case, the host materialmay be a hole-transporting material selected from the group ofsubstituted tri-aryl amines. The host material may be anelectron-transporting material selected from the group of metalquinoxolates, oxadiazoles, and triazoles. An example of a host materialis 4,4′-N,N′-dicarbazole-biphenyl (CBP), which has the formula:

The emissive layer may also contain a polarization molecule, present asa dopant in said host material and having a dipole moment, that affectsthe wavelength of light emitted when said emissive dopant moleculeluminesces.

A layer formed of an electron transporting material is used to transportelectrons into the emissive layer comprising the emissive molecule andthe (optional) host material. The electron transporting material may bean electron-transporting matrix selected from the group of metalquinoxolates, oxadiazoles, and triazoles. An example of an electrontransporting material is tris-(8-hydroxyquinoline) aluminum (Alq₃).

A layer formed of a hole transporting material is used to transportholes into the emissive layer comprising the emissive molecule and the(optional) host material. An example of a hole transporting material is4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl [“α-NPD”].

The use of an exciton blocking layer (“barrier layer”) to confineexcitons within the luminescent layer (“luminescent zone”) is greatlypreferred. For a hole-transporting host, the blocking layer may beplaced between the luminescent layer and the electron transport layer.An example of a material for such a barrier layer is2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproineor BCP), which has the formula:

For a situation with a blocking layer between a hole-conducting host andthe electron transporting layer (as is the case in Example 2 below), oneseeks the following characteristics, which are listed in order ofrelative importance.

-   -   1. The difference in energy between the LUMO and HOMO of the        blocking layer is greater than the difference in energy between        the triplet and ground state singlet of the host material.    -   2. Triplets in the host material are not quenched by the        blocking layer.    -   3. The ionization potential (IP) of the blocking layer is        greater than the ionization potential of the host. (Meaning that        holes are held in the host.)    -   4. The energy level of the LUMO of the blocking layer and the        energy level of the LUMO of the host are sufficiently close in        energy such that there is less than 50% change in the overall        conductivity of the device.    -   5. The blocking layer is as thin as possible subject to having a        thickness of the layer that is sufficient to effectively block        the transport of excitons from the emissive layer into the        adjacent layer.        That is, to block excitons and holes, the ionization potential        of the blocking layer should be greater than that of the HTL,        while the electron affinity of the blocking layer should be        approximately equal to that of the ETL to allow for facile        transport of electrons.        [For a situation in which the emissive (“emitting”) molecule is        used without a hole transporting host, the above rules for        selection of the blocking layer are modified by replacement of        the word “host” by “emitting molecule.”]

For the complementary situation with a blocking layer between aelectron-conducting host and the hole-transporting layer one seekscharacteristics (listed in order of importance):

-   -   1. The difference in energy between the LUMO and HOMO of the        blocking layer is greater than the difference in energy between        the triplet and ground state singlet of the host material.    -   2. Triplets in the host material are not quenched by the        blocking layer.    -   3. The energy of the LUMO of the blocking layer is greater than        the energy of the LUMO of the (electron-transporting) host.        (Meaning that electrons are held in the host.)    -   4. The ionization potential of the blocking layer and the        ionization potential of the host are such that holes are readily        injected from the blocker into the host and there is less than a        50% change in the overall conductivity of the device.    -   5. The blocking layer is as thin as possible subject to having a        thickness of the layer that is sufficient to effectively block        the transport of excitons from the emissive layer into the        adjacent layer.        [For a situation in which the emissive (“emitting”) molecule is        used without an electron transporting host, the above rules for        selection of the blocking layer are modified by replacement of        the word “host” by “emitting molecule.”]

The present invention covers articles of manufacture comprising OLEDscomprising a new family of phosphorescent materials, which can be usedas dopants in OLEDs, and methods of manufacturing the articles. Thesephosphorescent materials are cyclometallated platinum, iridium, orosmium complexes, which provide electroluminescent emission at awavelength between 400 nm and 700 nm. The present invention is furtherdirected to OLEDs that are capable of producing an emission that willappear blue, that will appear green, and that will appear red.

More specifically, OLEDs of the present invention comprise, for example,an emissive layer comprised of platinum (II) complexed withBis[2-(2-phenyl)pyridinato-N,C2], Bis[2-(2′-thienyl)pyridinato-N,C3],and Bis[benzo(h)quinolinato-N,C]. The compoundcis-Bis[2-(2′-thienyl)pyridinato-N,C3] Pt(II) gives a strong orange toyellow emission.

The invention is further directed to emissive layers wherein theemissive molecule is selected from the group of phosphorescentorganometallic complexes, wherein the emissive molecule containssubstituents selected from the class of electron donors and electronacceptors. The emissive molecule may be further selected from the groupof phosphorescent organometallic platinum, iridium or osmium complexesand may be still further selected from the group of phosphorescentcyclometallated platinum, iridium or osmium complexes, wherein theorganic molecule contains substituents selected from the class ofelectron donors and electron acceptors.

The invention is further directed to an organic light emitting devicecomprising a heterostructure for producing luminescence, wherein theemissive layer comprises a host material, an emissive molecule, presentas a dopant in said host material, adapted to luminesce when a voltageis applied across the heterostructure, wherein the emissive molecule isselected from the group consisting of cyclometallated platinum, iridiumor osmium complexes and wherein there is a polarization molecule,present as a dopant in the host material, which polarization moleculehas a dipole moment and which polarization molecule alters thewavelength of the luminescent light emitted by the emissive dopantmolecule. The polarization molecule may be an aromatic moleculesubstituted by electron donors and electron acceptors.

The present invention is directed to OLEDs, and a method of fabricatingOLEDs, in which emission from the device is obtained via aphosphorescent decay process wherein the phosphorescent decay rate israpid enough to meet the requirements of a display device. Morespecifically, the present invention is directed to OLEDs comprised of amaterial that is capable of receiving the energy from an exciton singletor triplet state and emitting that energy as phosphorescent radiation.

The OLEDs of the present invention may be used in substantially any typeof device which is comprised of an OLED, for example, in OLEDs that areincorporated into a larger display, a vehicle, a computer, a television,a printer, a large area wall, theater or stadium screen, a billboard ora sign.

The present invention is also directed to complexes of formula L L′ L″M, wherein L, L′, and L″ are distinct bidentate ligands and M is a metalof atomic number greater than 40 which forms an octahedral complex withthe three bidentate ligands and is preferably a member of the third row(of the transition series of the periodic table) transition metals, mostpreferably Ir and Pt. Alternatively, M can be a member of the second rowtransition metals, or of the main group metals, such as Zr and Sb. Someof such organometallic complexes electroluminesce, with emission comingfrom the lowest energy ligand or MLCT state. Such electroluminescentcompounds can be used in the emitter layer of organic light emittingdiodes, for example, as dopants in a host layer of an emitter layer inorganic light emitting diodes. This invention is further directed toorganometallic complexes of formula L L′ L″ M, wherein L, L′, and L″ arethe same (represented by L₃M) or different (represented by L L′ L″ M),wherein L, L′, and L″ are bidentate, monoanionic ligands, wherein M is ametal which forms octahedral complexes, is preferably a member of thethird row of transition metals, more preferably Ir or Pt, and whereinthe coordinating atoms of the ligands comprise sp² hybridized carbon anda heteroatom. The invention is further directed to compounds of formulaL₂MX, wherein L and X are distinct bidentate ligands, wherein X is amonoanionic bidentate ligand, wherein L coordinates to M via atoms of Lcomprising sp² hybridized carbon and heteroatoms, and wherein M is ametal forming an octahedral complex, preferably iridium or platinum. Itis generally expected that the ligand L participates more in theemission process than does X. The invention is directed to meridianalisomers of L₃M wherein the heteroatoms (such as nitrogen) of two ligandsL are in a trans configuration. In the embodiment in which M iscoordinated with an sp² hybridized carbon and a heteroatom of theligand, it is preferred that the ring comprising the metal M, the sp²hybridized carbon and the heteroatom contains 5 or 6 atoms. Thesecompounds can serve as dopants in a host layer which functions as aemitter layer in organic light emitting diodes.

Furthermore, the present invention is directed to the use of complexesof transition metal species M with bidentate ligands L and X incompounds of formula L₂MX in the emitter layer of organic light emittingdiodes. A preferred embodiment is compounds of formula L₂IrX, wherein Land X are distinct bidentate ligands, as dopants in a host layerfunctioning as an emitter layer in organic light emitting diodes.

The present invention is also directed to an improved synthesis oforganometallic molecules which function as emitters in light emittingdevices. These compounds of this invention can be made according to thereaction:L₂M(μ−Cl)₂ML₂+XH→L₂MX+HClwherein L₂M(μ−Cl)₂ML₂ is a chloride bridged dimer with L a bidentateligand, and M a metal such as Ir; XH is a Bronsted acid which reactswith bridging chloride and serves to introduce a bidentate ligand X,where XH can be, for example, acetylacetone, 2-picolinic acid, orN-methylsalicyclanilide, and H represents hydrogen. The method involvescombining the L₂M(μ−Cl)₂ML₂ chloride bridged dimer with the XH entity.The resultant product of the form L₂MX has approximate octahedraldisposition of the bidentate ligands L, L, and X about M.

The resultant compounds of formula L₂MX can be used as phosphorescentemitters in organic light emitting devices. For example, the compoundwherein L=(2-phenylbenzothiazole), X=acetylacetonate, and M=Ir (thecompound abbreviated as BTIr) when used as a dopant in4,4′-N,N′-dicarbazole-biphenyl (CBP) (at a level 12% by mass) to form anemitter layer in an OLED shows a quantum efficiency of 12%. Forreference, the formula for CBP is:

The synthetic process to make L₂MX compounds of the present inventionmay be used advantageously in a situation in which L, by itself, isfluorescent but the resultant L₂MX is phosphorescent. One specificexample of this is where L=coumarin-6.

The synthetic process of the present invention facilitates thecombination of L and X pairs of certain desirable characteristics. Forexample, the present invention is further directed to the appropriateselection of L and X to allow color tuning of the complex L₂MX relativeto L₃M. For example, Ir(ppy)₃ and (ppy)₂Ir(acac) both give strong greenemission with a λ_(max) of 510 nm (ppy denotes phenyl pyridine).However, if the X ligand is formed from picolinic acid instead of fromacetylacetone, there is a small blue shift of about 15 nm.

Furthermore, the present invention is also directed to a selection of Xsuch that it has a certain HOMO level relative to the L₃M complex sothat carriers (holes or electrons) might be trapped on X (or on L)without a deterioration of emission quality. In this way, carriers(holes or electrons) which might otherwise contribute to deleteriousoxidation or reduction of the phosphor would be impeded. The carrierthat is remotely trapped could readily recombine with the oppositecarrier either intramolecularly or with the carrier from an adjacentmolecule.

The present invention, and its various embodiments, are discussed inmore detail in the examples below. However, the embodiments may operateby different mechanisms. Without limitation and without limiting thescope of the invention, we discuss the different mechanisms by whichvarious embodiments of the invention may operate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Electronic absorbance spectra of Pt(thpy)₂, Pt(thq)₂, andPt(bph)(bpy).

FIG. 2. Emission spectra of Pt(thpy)₂, Pt(thq)₂, and Pt(bph)(bpy).

FIG. 3. Energy transfer from polyvinylcarbazole (PVK) to Pt(thpy)₂ inthe solid film.

FIG. 4. Characteristics of OLED with Pt(thpy)₂ dopant:

-   -   (a) I-V characteristic;    -   (b) Light output curve.

FIG. 5. Quantum efficiency dependence on applied voltage for OLED withPt(thpy)₂ dopant.

FIG. 6. Characteristics of the OLED device with Pt(thpy)₂ dopant:

-   -   (a) normalized electroluminescence (EL) spectrum of the device        at 22 V;    -   (b) CIE diagram based on normalized EL spectrum.

FIG. 7. Proposed energy level structure of the electrophosphorescentdevice of Example 2. The highest occupied molecular orbital (HOMO)energy and the lowest unoccupied molecular orbital (LUMO) energy areshown (see I. G. Hill and A. Kahn, J. Appl. Physics (1999)). Note thatthe HOMO and LUMO levels for Ir(ppy)₃ are not known. The bottom portionof FIG. 7 shows structural chemical formulae for:

-   -   (a) Ir(ppy)₃;    -   (b) CBP; and    -   (c) BCP.

FIG. 8. The external quantum efficiency of OLEDs using Ir(ppy)₃: CBPluminescent layers. Peak efficiencies are observed for a mass ratio of6% Ir(ppy)₃ to CBP. The 100% Ir(ppy)₃ device has a slightly differentstructure than shown in FIG. 7. In it, the Ir(ppy)₃ layer is 300 Å thickand there is no BCP blocking layer. The efficiency of a 6% Ir(ppy)₃: CBPdevice grown without a BCP layer is also shown.

FIG. 9. The power efficiency and luminance of the 6% Ir(ppy)₃: CBPdevice. At 100 cd/m², the device requires 4.3 V and its power efficiencyis 19 lm/W.

FIG. 10. The electroluminescent spectrum of 6% Ir(ppy)₃: CBP. Inset: TheCommission Internationale de L'Eclairage (CIE) chromaticity coordinatesof Ir(ppy)₃ in CBP are shown relative to fluorescent green emitters Alq₃and poly(p-phenylenevinylene) (PPV).

FIG. 11. Expected structure of L₂IrX complexes along with the structureexpected for PPIr. Four examples of X ligands used for these complexesare also shown. The structure shown is for an acac derivative. For theother X type ligands, the O—O ligand would be replaced with an N—Oligand.

FIG. 12. Comparison of facial and meridianal isomers of L₃M.

FIG. 13. Molecular formulae of mer-isomers disclosed herewith:mer-Ir(ppy)₃ and mer-Ir(bq)₃. PPY (or ppy) denotes phenyl pyridyl and BQ(or bq) denotes 7,8-benzoquinoline.

FIG. 14. Models of mer-Ir(ppy)₃ (left) and (ppy)₂Ir(acac) (right).

FIG. 15. (a) Electroluminescent device data (quantum efficiency vs.current density) for 12% by mass “BTIr” in CBP. BTIr stands forbis(2-phenylbenzothiazole)iridium acetylacetonate; (b) Emission spectrumfrom same device FIG. 16. Representative molecule to trap holes (L₂IrXcomplex).

FIG. 17. Emission spectrum of Ir(3-MeOppy)₃.

FIG. 18. Emission spectrum of tpyIrsd.

FIG. 19. Proton NMR spectrum of tpyIrsd (=typIrsd).

FIG. 20. Emission spectrum of thpyIrsd.

FIG. 21. Proton NMR spectrum of thpyIrsd.

FIG. 22. Emission spectrum of btIrsd.

FIG. 23. Proton NMR spectrum of btIrsd.

FIG. 24. Emission spectrum of BQIr.

FIG. 25. Proton NMR spectrum of BQIr.

FIG. 26. Emission spectrum of BQIrFA.

FIG. 27. Emission spectrum of THIr (=thpy; THPIr).

FIG. 28. Proton NMR spectrum of THPIr.

FIG. 29. Emission spectra of PPIr.

FIG. 30. Proton NMR spectrum of PPIr.

FIG. 31. Emission spectrum of BTHPIr (=BTPIr).

FIG. 32. Emission spectrum of tpyIr.

FIG. 33. Crystal structure of tpyIr showing trans arrangement ofnitrogen.

FIG. 34. Emission spectrum of C6.

FIG. 35. Emission spectrum of C6Ir.

FIG. 36. Emission spectrum of PZIrP.

FIG. 37. Emission spectrum of BONIr.

FIG. 38. Proton NMR spectrum of BONIr.

FIG. 39. Emission spectrum of BTIr.

FIG. 40. Proton NMR spectrum of BTIr.

FIG. 41. Emission spectrum of BOIr.

FIG. 42. Proton NMR spectrum of BOIr.

FIG. 43. Emission spectrum of BTIrQ.

FIG. 44. Proton NMR spectrum of BTIrQ.

FIG. 45. Emission spectrum of BTIrP.

FIG. 46. Emission spectrum of BOIrP.

FIG. 47. Emission spectrum of btIr-type complexes with differentligands.

FIG. 48. Proton NMR spectrum of mer-Irbq.

FIG. 49. Other suitable L and X ligands for L₂MX compounds. In all ofthese ligands listed, one can easily substitute S for O and still have agood ligand.

FIG. 50. Examples of L L′ L″ M compounds. In the listed examples of L L′L″ M and L L′ M X compounds, the compounds would be expected to emitfrom the lowest energy ligand or the MLCT state, involving the bq orthpy ligands. In the listed example of an L M X X′ compound, emissiontherefrom is expected from the ppy ligand. The X and X′ ligands willmodify the physical properties (for example, a hole trapping group couldbe added to either ligand).

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to emissive molecules, whichluminesce when a voltage is applied across a heterostructure of anorganic light-emitting device and which molecules are selected from thegroup of phosphorescent organometallic complexes, and to structures, andcorrelative molecules of the structures, that optimize the emission ofthe light-emitting device. The term “organometallic” is as generallyunderstood by one of ordinary skill, as given, for example, in“Inorganic Chemistry” (2nd edition) by Gary L. Miessler and Donald A.Tarr, Prentice-Hall (1998). The invention is further directed toemissive molecules within the emissive layer of an organiclight-emitting device which molecules are comprised of phosphorescentcyclometallated platinum, iridium, or osmium complexes. Onelectroluminescence, molecules in this class may produce emission whichappears red, blue, or green. Discussions of the appearance of color,including descriptions of CIE charts, may be found in H. Zollinger,Color Chemistry, VCH Publishers, 1991 and H. J. A. Dartnall, J. K.Bowmaker, and J. D. Mollon, Proc. Roy. Soc. B (London), 1983, 220,115-130.

The present invention will now be described in detail for specificpreferred embodiments of the invention, it being understood that theseembodiments are intended only as illustrative examples and the inventionis not to be limited thereto.

Synthesis of the Cyclometallated Platinum Complexes

We have synthesized a number of different Pt cyclometallated complexes.

Numerous publications, reviews, and books are dedicated to the chemistryof cyclometallated compounds, which also are calledintramolecular-coordination compounds. (I. Omae, OrganometallicIntramolecular-coordination compounds. N.Y. 1986. G. R. Newkome, W. E.Puckett, V. K. Gupta, G. E. Kiefer, Chem. Rev. 1986, 86, 451. A. D.Ryabov, Chem. Rev. 1990, 90, 403). Most of the publications depictmechanistical aspects of the subject and primarily on thecyclometallated compounds with one bi- or tri-dentate ligand bonded tometal by C-M single bond and having cycle closed with one or two otherX-M bonds where X may be N, S, P, As, O, Not so much literature wasdevoted to bis- or tris-cyclometallated complexes, which do not possessany other ligands but C,N type bi-dentate ones. Some of the subject ofthis invention is in these compounds because they are not only expectedto have interesting photochemical properties as most cyclometallatedcomplexes do, but also should exhibit increased stability in comparisonwith their monocyclometallated analogues. Most of the work onbis-cyclopaladated and bis-cycloplatinated compounds was performed byvon Zelewsky et al. (For a review see: M. Maestri, V. Balzani, Ch.Deuschel-Cornioley, A. von Zelewsky, Adv. Photochem. 1992 17, 1. L.Chassot, A. Von Zelewsky, Helv. Chim. Acta 1983, 66, 243. L. Chassot, E.Muler, A. von Zelewsky, Inorg. Chem. 1984, 23, 4249. S Bonafede, M.Ciano, F. Boletta, V. Balzani, L. Chassot, A. von Zelewsky, J. Phys.Chem. 1986, 90, 3836. L. Chassot, A. von Zelewsky, D. Sandrini, M.Maestri, V. Balzani, J. Am. Chem. Soc. 1986, 108, 6084. Ch.Cornioley-Deuschel, A. von Zelewsky, Inorg. Chem. 1987, 26, 3354. L.Chassot, A. von Zelewsky, Inorg. Chem. 1987, 26, 2814. A. von Zelewsky,A. P. Suckling, H. Stoeckii-Evans, Inorg. Chem. 1993, 32, 4585. A. vonZelewsky, P. Belser, P. Hayoz, R. Dux, X. Hua, A. Suckling, H.Stoeckii-Evans, Coord. Chem. Rev. 1994, 132, 75. P. Jolliet, M. Gianini,A. von Zelewsky, G. Bernardinelli, H. Stoeckii-Evans, Inorg. Chem. 1996,35, 4883. H. Wiedenhofer, S. Schutzenmeier, A. von Zelewsky, H. Yersin,J. Phys. Chem. 1995, 99, 13385. M. Gianini, A. von Zelewsky, H.Stoeckii-Evans, Inorg. Chem. 1997, 36, 6094.) In one of their earlyworks, (M. Maestri, D. Sandrini, V. Balzani, L. Chassot, P. Jolliet, A.von Zelewsky, Chem. Phys. Lett. 1985, 122, 375), luminescent propertiesof three bis-cycloplatinated complexes were investigated in detail. Thesummary of the previously reported results on Pt bis-cyclometallatedcomplexes important for our current research is as follows:

-   -   i. in general, cyclometallated complexes having a 5-membered        ring formed between the metal atom and C,X ligand are more        stable.    -   ii. from the point of view of stability of resulting compounds,        complexes not containing anionic ligands are preferred; thus,        bis-cyclometallated complexes are preferred to        mono-cyclometallated ones.    -   iii. a variety of Pt(Pd) cyclometallated complexes were        synthesized, homoleptic (containing similar C,X ligands),        heteroleptic (containing two different cyclometallating C,X        ligands) and complexes with one C,C cyclometallating ligand and        one N,N coordinating ligand.    -   iv. most bis-cyclometallated complexes show M⁺ ions upon        electron impact ionization in their mass spectra; this can be a        base for our assumption on their stability upon vacuum        deposition.    -   v. on the other hand, some of the complexes are found not to be        stable in certain solvents; they undergo oxidative addition        reactions leading to Pt(IV) or Pd(IV) octahedral complexes.    -   vi. optical properties are reported only for some of the        complexes; mostly absorption data is presented. Low-energy        electron transitions observed in both their absorption and        emission spectra are assigned to MLCT transitions.

vii. reported luminescent properties are summarized in Table 1. Usedabbreviations are explained in Scheme 1. Upon transition frombis-cyclometalated complexes with two C,N ligands to the complexes withone C,C and one N,N ligand batochromic shift in emission was observed.(M. Maestri, D. Sandrini, V. Balzani, A. von Zelewsky, C.Deuschel-Cornioley, P. Jolliet, Helv. Chim. Acta 1988, 71, 1053. TABLE 1Absorption and emission properties of several cycloplatinated complexes.Reproduced from A.von Zelewsky et. al (Chem. Phys. Lett., 1985, 122, 375and Helv. Chim. Acta 1988, 17, 1053). Abbreviation explanations aregiven in Scheme 1. emission spectra absorption 77 K 293 K solventλmax(ε) λmax(τ) λmax(τ) Pt(Phpy)₂(1) CH₃CN 402(12800) 491(4.0) —291(27700) Pt(Thpy)₂(2) CH₃CN 418(10500) 570(12.0) 578(2.2) 303(26100)Pt(Bhq)₂(3) CH₃CN 421(9200) 492(6.5) — 367(12500) 307(15000)Pt(bph)(bpy)(4) Scheme 1: Explanations for abbreviations used in table1.

We synthesized different bis-cycloplatinated complexes in order toinvestigate their optical properties in different hosts, both polymericand molecular, and utilize them as dopants in corresponding hosts fororganic light-emitting diodes (OLEDs). Usage of the complexes inmolecular hosts in OLEDs prepared in the vacuum deposition processrequires several conditions to be satisfied. The complexes should besublimable and stable at the standard deposition conditions (vacuum˜10⁻⁶ torr). They should show emission properties interesting for OLEDapplications and be able to accept energy from host materials used, suchas Alq₃ or NPD. On the other hand, in order to be useful in OLEDsprepared by wet techniques, the complexes should form true solutions inconventional solvents (e.g., CHCl₃) with a wide range of concentrationsand exhibit both emission and efficient energy transfer from polymerichosts (e.g., PVK). All these properties of cycloplatinated complexeswere tested. In polymeric hosts we observe efficient luminescence fromsome of the materials.

Syntheses proceeded as follows:

2-(2-thienyl)pyridine. Synthesis is shown in Scheme 2, and was performedaccording to procedure close to the published one (T. Kauffmann, A.Mitschker, A. Woltermann, Chem. Ber. 1983, 116, 992). For purificationof the product, instead of recommended distillation, zonal sublimationwas used (145-145-125° C., 2-3 hours). Light brownish white solid (yield69%). Mass-spec: m/z: 237(18%), 161 (100%, M⁺), 91 (71%).

¹H NMR (250 MHZ, DMSO-d₆) δ, ppm: 6.22-6.28 (d. of d., 1H), 6.70-6.80(d. of d., 1H), 6.86-7.03 (m, 3H), 7.60-7.65 (m, 1H). ¹³C NMR (250 MHZ,DMSO-d₆): 118.6, 122.3, 125.2, 128.3, 128.4, 137.1, 144.6, 149.4, 151.9.

2-(2-thienyl)quinoline. Synthesis is displayed in Scheme 3, and was madeaccording to published procedure (K. E. Chippendale, B. Iddon, H.Suschitzky, J. Chem. Soc. 1949, 90, 1871). Purification was made exactlyfollowing the literature, as neither sublimation nor columnchromatography gave as good results as recrystallizations from (a)petroleum ether, and (b) EtOH—H₂O (1:1) mixture. Pale yellow solid, getsmore yellow with time (yield 84%). Mass-spec: m/z: 217 (32%), 216 (77%),215 (83%), 214 (78%), 213 (77%), 212 (79%), 211(100%, M⁺), 210 (93%),209 (46%). ¹H NMR (250 MHZ, DMSO-d₆) δ, ppm: 7.18-7.24 (d. of d., 1H),7.48-7.58 (d. of d. of d., 1H), 7.67-7.78 (m, 2H), 7.91-7.97 (m, 3H),8.08-8.11 (d, 1H), 8.36-8.39 (d, 1H).

2-(2′-bromophenyl)pyridine. Synthesis was performed according toliterature (D. H. Hey, C. J. M. Stirling, G. H. Williams, J. Chem. Soc.1955, 3963; R. A. Abramovich, J. G. Saha, J. Chem. Soc. 1964, 2175). Itis outlined in Scheme 4. Literature on the subject was dedicated to thestudy of aromatic substitution in different systems, including pyridine,and study of isomeric ratios in the resulting product. Thus in order toresolve isomer mixtures of different substituted phenylpyridines, not2-(2′-bromophenyl)pyridine, the authors utilized 8 ft.×¼ in. columnpacked with ethylene glycol succinate (10%) on Chromosorb W at 155° C.and some certain helium inlet pressure. For resolving the reactionmixture we obtained, we used column chromatography with hexanes:THF(1:1) and haxanes:THF:PrOH-1 (4:4:1) mixtures as eluents on silica gelbecause this solvent mixture gave best results in TLC (three wellresolved spots). Only the first spot in the column gave mass spec majorpeak corresponding to n-(2′-bromophenyl)pyridines (m/z: 233, 235), inthe remaining spots this peak was minor. Mass spec of the firstfraction: m/z: 235 (97%), 233 (100%, M⁺), 154 (86%), 127 (74%). ¹H NMRof the first fraction (250 MHZ, DMSO-d6) δ, ppm: 7.27-7.51 (m, 4H),7.59-7.96 (m, 2H), 8.57-8.78 (m, 2H).

Sublimation of the 1^(st) fraction product after column did not lead todisappearance of the peaks of contaminants in ¹H NMR spectrum, and we donot expect the sublimation to lead to resolving the isomers if present.

2-phenylpyridine. Was synthesized by literature procedure (J. C. W.Evans, C. F. H. Allen, Org. Synth. Cell. 1943, 2, 517) and is displayedin Scheme 5. Pale yellow oil darkening in the air (yield 48%). ¹H NMR(250 MHZ, DMSO-d₆) of the product after vacuum distillation: δ, ppm:6.70-6.76 (m, 1H), 6.92-7.10 (m, 3H), 7.27-7.30 (m, 1H), 7.36-7.39 (q,1H), 7.60-7.68 (m, 2H), 8.16-8.23 (m, 1H)).

2,2′-diaminobiphenyl. Was prepared by literature method (R. E. Moore, A.Furst, J. Org. Chem. 1958, 23, 1504) (Scheme 6). Pale pink solid (yield69%). ¹H NMR (250 MHZ, DMSO-d₆) δ, ppm: 5.72-5.80 (t. of d., 2H),5.87-5.93 (d. of d., 2H), 6.03-6.09 (d. of d., 2H), 6.13-6.23 (t. of d.,2H). Mass spec: m/z: 185 (40%), 184 (100%, M⁺), 183 (73%), 168 (69%),167 (87%), 166(62%), 139 (27%).

2,2′-dibromobiphenyl. (Scheme 6) (A. Uehara, J. C. Bailar, Jr., J.Organomet. Chem. 1982, 239, 1).

2,2′-dibromo-1,1′-binaphthyl. Was synthesized according to literature(H. Takaya, S. Akutagawa, R. Noyori, Org. Synth. 1989, 67, 20) (Scheme7).

trans-Dichloro-bis-(diethyl sulfide) platinum (II). Prepared by apublished procedure (G. B. Kauffman, D. O. Cowan, Inorg. Synth. 1953, 6,211) (Scheme 8). Bright yellow solid (yield 78%).

cis-Dichloro-bis-(diethyl sulfide) platinum (II). Prepared by apublished procedure (G. B. Kauffman, D. O. Cowan, Inorg. Synth. 1953, 6,211). (Scheme 8). Yellow solid (63%).

cis-Bis[2-(2′-thienyl)pyridinato-N,C^(5′)platinum (II). Was synthesizedaccording to literature methods (L. Chassot, A. von Zelewsky, Inorg.Chem. 1993, 32, 4585). (Scheme 9). Bright red crystals (yield 39%). Massspec: m/z: 518 (25%), 517 (20%), 516 (81%), 513 (100%,M⁺), 514 (87%),481 (15%), 354 (23%).

cis-Bis[2-(2′-thienyl)quinolinato-N,C³)platinum (II). Was preparedfollowing published procedures (P. Jolliet, M. Gianini, A. von Zelewsky,G. Bernardinelli, H. Stoeckii-Evans, Inorg. Chem. 1996, 35, 4883).(Scheme 10). Dark red solid (yield 21%).

Absorption spectra were recorded on AVIV Model 14DS-UV-Vis-IRspectrophotometer and corrected for background due to solventabsorption. Emission spectra were recorded on PTI QuantaMaster ModelC-60SE spectrometer with 1527 PMT detector and corrected for detectorsensitivity inhomogeneity.

Vacuum deposition experiments were performed using standard high vacuumsystem (Kurt J. Lesker vacuum chamber) with vacuum ˜10⁻⁶ torr. Quartzplates (ChemGlass Inc.) or borosilicate glass-IndiumTin Oxide plates(ITO, Delta Technologies, Lmtd.), if used as substrates for deposition,were pre-cleaned according to the published procedure for the later (A.Shoustikov, Y. You, P. E. Burrows, M. E. Thomspon, S. R. Forrest, Synth.Met. 1997, 91, 217).

Thin film spin coating experiments were done with standard spin coater(Specialty Coating Systems, Inc.) with regulatable speed, accelerationspeed, and deceleration speed. Most films were spun coat with 4000 RPMspeed and maximum acceleration and deceleration for 40 seconds.

Optical properties of the Pt cyclometalated complexes are shown above inTable 1.

Optical Properties in Solution:

Absorbance spectra of the complexes Pt(thpy)₂, Pt(thq)₂ and Pt(bph)(bpy)in solution (CHCl₃ or CH₂Cl₂) were normalized and are presented inFIG. 1. Absorption maximum for Pt(phpy)₂ showed a maximum at ca. 400 nm,but because the complex apparently requires further purification, thespectrum is not presented.

Normalized emission spectra are shown in FIG. 2. Excitation wavelengthsfor Pt(thpy)₂, Pt(thq)₂ and Pt(bph)(bpy) are correspondingly 430 nm, 450nm, and 449 nm (determined by maximum values in their excitationspectra). Pt(thpy)₂ gives strong orange to yellow emission, whilePt(thq)₂ gives two lines at 500 and 620 nm. The emission form thesematerials is due to efficient phosphorescence. Pt(bph)(bpy) gives blueemission, centered at 470 nm. The emission observed for Pt(bph)(bpy) ismost likely due to fluorescence and not phosphorescence.

Emission Lifetimes and Quantum Yields in Solution: Pt(thPy)₂: 3.7 μs(CHCl₃, deoxygenated for 10 min) 0.27 Pt(thq)₂: 2.6 μs (CHCl₃,deoxygenated for 10 min) not measured Pt(bph)(bpy): not in μs region(CH₂O₂, not measured deoxygenated for 10 min)Optical Properties in PS Solid Matrix:

Pt(thpy)₂: Emission maximum is at 580 nm (lifetime 6.5 μs) uponexcitation at 400 nm. Based on the increased lifetime for the sample inpolystyrene we estimate a quantum efficiency in polystyrene forPt(thpy)₂ of 0.47.

Pt(thq)₂: Emission maximum at 608 nm (lifetime 7.44 μs) upon excitationat 450 nm.

Optical Properties of the Complexes in PVK Film:

These measurements were made for Pt(thpy)₂ only. Polyvinylcarbazole(PVK) was excited at 250 nm and energy transfer from PVK to Pt(thpy)₂was observed (FIG. 3). The best weight PVK:Pt(thpy)₂ ratio for theenergy transfer was found to be ca. 100:6.3.

EXAMPLES OF LIGHT EMITTING DIODES Example 1 ITO/PVK:PBD.Pt(thpy)₂(100:40:2)/Ag:Mg/Ag

Pt(thpy)₂ does not appear to be stable toward sublimation. In order totest it in an OLED we have fabricated a polymer blended OLED withPt(thpy)₂ dopant. The optimal doping level was determined by thephotoluminescence study described above. The emission from this devicecomes exclusively from the Pt(thpy)₂ dopant. Typical current-voltagecharacteristic and light output curve of the device are shown in FIG. 4.Quantum efficiency dependence on applied voltage is demonstrated in FIG.5. Thus, at 22 V quantum efficiency is ca. 0.11%. The high voltagerequired to drive this device is a result of the polymer blend OLEDstructure and not the dopant. Similar device properties were observedfor a polymer blend device made with a coumarin dopant in place ofPt(thpy)₂. In addition, electroluminescence spectrum and CIE diagram areshown in FIG. 6.

Example 2

In this example, we describe OLEDs employing the green,electrophosphorescent material fac tris(2-phenylpyridine) iridium(Ir(ppy)₃). This compound has the following formulaic representation:

The coincidence of a short triplet lifetime and reasonablephotoluminescent efficiency allows Ir(ppy)₃-based OLEDs to achieve peakquantum and power efficiencies of 8.0% (28 cd/Å) and ˜30 lm/Wrespectively. At an applied bias of 4.3V, the luminance reaches 100cd/m² and the quantum and power efficiencies are 7.5% (26 cd/Å) and 19lm/W, respectively.

Organic layers were deposited by high vacuum (10⁻⁶ Torr) thermalevaporation onto a cleaned glass substrate precoated with transparent,conductive indium tin oxide. A 400 Å thick layer of4,4′-bis(N-(1-naphthyl)-N-phenyl-amino)biphenyl (α-NPD) is used totransport holes to the luminescent layer consisting of Ir(ppy)₃ in CBP.A 200 Å thick layer of the electron transport materialtris-(8-hydroxyquinoline) aluminum (Alq₃) is used to transport electronsinto the Ir(ppy)₃:CBP layer, and to reduce Ir(ppy)₃ luminescenceabsorption at the cathode. A shadow mask with 1 mm diameter openings wasused to define the cathode consisting of a 1000 Å thick layer of 25:1Mg:Ag, with a 500 Å thick Ag cap. As previously (O'Brien, et al., App.Phys. Lett. 1999, 74, 442-444), we found that a thin (60 Å) barrierlayer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine,or BCP) inserted between the CBP and the Alq₃ was necessary to confineexcitons within the luminescent zone and hence maintain highefficiencies. In O'Brien et al., Appl. Phys. Lett. 1999, 74, 442-444, itwas argued that this layer prevents triplets from diffusing outside ofthe doped region. It was also suggested that CBP may readily transportholes and that BCP may be required to force exciton formation within theluminescent layer. In either case, the use of BCP clearly serves to trapexcitons within the luminescent region. The molecular structuralformulae of some of the materials used in the OLEDs, along with aproposed energy level diagram, is shown in FIG. 7.

FIG. 8 shows the external quantum efficiencies of several Ir(ppy)3-basedOLEDs. The doped structures exhibit a slow decrease in quantumefficiency with increasing current. Similar to the results for theAlq₃:PtOEP system, the doped devices achieve a maximum efficiency (˜8%)for mass ratios of Ir(ppy)₃:CBP of approximately 6-8%. Thus, the energytransfer pathway in Ir(ppy)₃:CBP is likely to be similar to that inPtOEP:Alq₃ (Baldo, et al., Nature, 1998, 395, 151; O'Brien, 1999, op.cit.) i.e. via short range Dexter transfer of triplets from the host. Atlow Ir(ppy)₃ concentrations, the lumophores often lie beyond the Dextertransfer radius of an excited Alq₃ molecule, while at highconcentrations, aggregate quenching is increased. Note thatdipole-dipole (Förster) transfer is forbidden for triplet transfer, andin the PtOEP:Alq₃ system direct charge trapping was not found to besignificant.

Example 3

In addition to the doped device, we fabricated a heterostructure wherethe luminescent region was a homogeneous film of Ir(ppy)₃. The reductionin efficiency (to ˜0.8%) of neat Ir(ppy)₃ is reflected in the transientdecay, which has a lifetime of only ˜100 ns, and deviates significantlyfrom mono-exponential behavior. A 6% Ir(ppy)₃:CBP device without a BCPbarrier layer is also shown together with a 6% Ir(ppy)₃:Alq₃ device witha BCP barrier layer. Here, very low quantum efficiencies are observed toincrease with current. This behavior suggests a saturation ofnonradiative sites as excitons migrate into the Alq₃, either in theluminescent region or adjacent to the cathode.

Example 4

In FIG. 9 we plot luminance and power efficiency as a function ofvoltage for the device of Example 2. The peak power efficiency is ˜30lm/W with a quantum efficiency of 8%, (28 cd/Å). At 100 cd/m², a powerefficiency of 19 lm/W with a quantum efficiency of 7.5% (26 cd/Å) isobtained at a voltage of 4.3V. The transient response of Ir(ppy)₃ in CBPis a mono-exponential phosphorescent decay of ˜500 ns, compared with ameasured lifetime (e.g., King, et al., J. Am. Chem. Soc., 1985, 107,1431-1432) of 2 μs in degassed toluene at room temperature. Theselifetimes are short and indicative of strong spin-orbit coupling, andtogether with the absence of Ir(ppy)₃ fluorescence in the transientresponse, we expect that Ir(ppy)₃ possesses strong intersystem crossingfrom the singlet to the triplet state. Thus all emission originates fromthe long lived triplet state. Unfortunately, slow triplet relaxation canform a bottleneck in electrophosphorescence and one principal advantageof Ir(ppy)₃ is that it possesses a short triplet lifetime. Thephosphorescent bottleneck is thereby substantially loosened. Thisresults in only a gradual decrease in efficiency with increasingcurrent, leading to a maximum luminance of ˜100,000 cd/m².

Example 5

In FIG. 10, the emission spectrum and Commission Internationale deL'Eclairage (CIE) coordinates of Ir(Ppy)₃ are shown for the highestefficiency device. The peak wavelength is λ=510 nm and the full width athalf maximum is 70 nm. The spectrum and CIE coordinates (x=0.27, y=0.63)are independent of current. Even at very high current densities (˜100mA/cm²) blue emission from CBP is negligible—an indication of completeenergy transfer.

Other techniques known to one of ordinary skill may be used inconjunction with the present invention. For example, the use of LiFcathodes (Hung, et al., Appl. Phys. Lett., 1997, 70, 152-154), shapedsubstrates (G. Gu, et al., Optics Letters, 1997, 22, 396-398), and novelhole transport materials that result in a reduction in operating voltageor increased quantum efficiency (B. Kippelen, et al., MRS (SanFrancisco, Spring, 1999) are also applicable to this work. These methodshave yielded power efficiencies of ˜20 lm/W in fluorescent smallmolecule devices (Kippelen, Id.). The quantum efficiency in thesedevices (Kido and Iizumi, App. Phys. Lett., 1998, 73, 2721) at 100 cd/m²is typically≦4.6% (lower than that of the present invention), and hencegreen-emitting electrophosphorescent devices with power efficienciesof >40 lm/W can be expected. Purely organic materials (Hoshino andSuzuki, Appl. Phys. Lett., 1996, 69, 224-226) may sometimes possessinsufficient spin orbit coupling to show strong phosphorescence at roomtemperature. While one should not rule out the potential of purelyorganic phosphors, the preferred compounds may be transition metalcomplexes with aromatic ligands. The transition metal mixes singlet andtriplet states, thereby enhancing intersystem crossing and reducing thelifetime of the triplet excited state.

The present invention is not limited to the emissive molecule of theexamples. One of ordinary skill may modify the organic component of theIr(ppy)₃ (directly below) to obtain desirable properties.

One may have alkyl substituents or alteration of the atoms of thearomatic structure.

These molecules, related to Ir(ppy)₃, can be formed from commerciallyavailable ligands. The R groups can be alkyl or aryl and are preferablyin the 3, 4, 7 and/or 8 positions on the ligand (for steric reasons).The compounds should give different color emission and may havedifferent carrier transport rates. Thus, the modifications to the basicIr(ppy)₃ structure in the three molecules can alter emissive propertiesin desirable ways.

Other possible emitters are illustrated below, by way of example.

This molecule is expected to have a blue-shifted emission compared toIr(ppy)₃. R and R′ can independently be alkyl or aryl.

Organometallic compounds of osmium may also be used in this invention.Examples include the following.

These osmium complexes will be octahedral with 6d electrons(isoelectronic with the Ir analogs) and may have good intersystemcrossing efficiency. R and R′ are independently selected from the groupconsisting of alkyl and aryl. They are believed to be unreported in theliterature.

Herein, X can be selected from the group consisting of N or P. R and R′are independently selected from the group alkyl and aryl.

The molecule of the hole-transporting layer of Example 2 is depictedbelow.

The present invention will work with other hole-transporting moleculesknown by one of ordinary skill to work in hole transporting layers ofOLEDs.

The molecule used as the host in the emissive layer of Example 2 isdepicted below.

The present invention will work with other molecules known by one ofordinary skill to work as hosts of emissive layers of OLEDs. Forexample, the host material could be a hole-transporting matrix and couldbe selected from the group consisting of substituted tri-aryl amines andpolyvinylcarbazoles.

The molecule used as the exciton blocking layer of Example 2 is depictedbelow. The invention will work with other molecules used for the excitonblocking layer, provided they meet the requirements listed in thesummary of the invention.

Molecules which are suitable as components for an exciton blocking layerare not necessarily the same as molecules which are suitable for a holeblocking layer. For example, the ability of a molecule to function as ahole blocker depends on the applied voltage, the higher the appliedvoltage, the less the hole blocking ability. The ability to blockexcitons is roughly independent of the applied voltage.

This invention is further directed to the synthesis and use of certainorganometallic molecules of formula L₂MX which may be doped into a hostphase in an emitter layer of an organic light emitting diode.Optionally, the molecules of formula L₂MX may be used at elevatedconcentrations or neat in the emitter layer. This invention is furtherdirected to an organic light emitting device comprising an emitter layercomprising a molecule of the formula L₂MX wherein L and X areinequivalent, bidentate ligands and M is a metal, preferably selectedfrom the third row of the transition elements of the periodic table, andmost preferably Ir or Pt, which forms octahedral complexes, and whereinthe emitter layer produces an emission which has a maximum at a certainwavelength λ_(max). The general chemical formula for these moleculeswhich are doped into the host phase is L₂MX, wherein M is a transitionmetal ion which forms octahedral complexes, L is a bidentate ligand, andX is a distinct bidentate ligand. Examples of L are2-(1-naphthyl)benzoxazole)), (2-phenylbenzoxazole),(2-phenylbenzothiazole), (2-phenylbenzothiazole), (7,8-benzoquinoline),coumarin, (thienylpyridine), phenylpyridine, benzothienylpyridine,3-methoxy-2-phenylpyridine, thienylpyridine, and tolylpyridine. Examplesof X are acetylacetonate (“acac”), hexafluoroacetylacetonate,salicylidene, picolinate, and 8-hydroxyquinolinate. Further examples ofL and X are given in FIG. 49 and still further examples of L and X maybe found in Comprehensive Coordination Chemistry, Volume 2, G. Wilkinson(editor-in-chief), Pergamon Press, especially in chapter 20.1 (beginningat page 715) by M. Calligaris and L. Randaccio and in chapter 20.4(beginning at page 793) by R. S. Vagg.

Synthesis of Molecules of Formula L₂MX

The compounds of formula L₂MX can be made according to the reaction:L₂M(μ−Cl)₂ML₂+XH→L₂MX+HClwherein L₂M(μ−Cl)₂ML₂ is a chloride bridged dimer with L a bidentateligand, and M a metal such as Ir; XH is a Bronsted acid which reactswith bridging chloride and serves to introduce a bidentate ligand X,wherein XH can be, for example, acetylacetone, hexafluoroacetylacetone,2-picolinic acid, or N-methylsalicyclanilide; and L₂MX has approximateoctahedral disposition of the bidentate ligands L, L, and X about M.

L₂Ir(m−Cl)₂IrL₂ complexes were prepared from IrCl₃.nH₂O and theappropriate ligand by literature procedures (S. Sprouse, K. A. King, P.J. Spellane, R. J. Watts, J. Am. Chem. Soc., 1984, 106, 6647-6653; forgeneral reference: G. A. Carlson, et al., Inorg. Chem., 1993, 32, 4483;B. Schmid, et al., Inorg. Chem., 1993, 33, 9; F. Garces, et al.; Inorg.Chem., 1988, 27, 3464; M. G. Colombo, et al., Inorg. Chem., 1993, 32,3088; A. Mamo, et al., Inorg. Chem., 1997, 36, 5947; S. Serroni, et al.;J. Am. Chem. Soc., 1994, 116, 9086; A. P. Wilde, et al., J. Phys. Chem.,1991, 95, 629; J. H. van Diemen, et al., Inorg. Chem., 1992, 31, 3518;M. G. Colombo, et al., Inorg. Chem., 1994, 33, 545), as described below.

Ir(3-MeOppy)₃. Ir(acac)₃ (0.57 g, 1.17 mmol) and3-methoxy-2-phenylpyridine (1.3 g, 7.02 mmol) were mixed in 30 ml ofglycerol and heated to 200° C. for 24 hrs under N₂. The resultingmixture was added to 100 ml of 1 M HCl. The precipitate was collected byfiltration and purified by column chromatography using CH₂Cl₂ as theeluent to yield the product as bright yellow solids (0.35 g, 40%). MS(EI): m/z (relative intensity) 745 (M⁺, 100), 561 (30), 372 (35).Emission spectrum in FIG. 17.

tpyIrsd. The chloride bridge dimer (tpyIrCl)₂ (0.07 g, 0.06 mmol),salicylidene (0.022 g, 0.16 mmol) and Na₂CO₃ (0.02 g, 0.09 mmol) weremixed in 10 ml of 1,2-dichloroethane and 2 ml of ethanol. The mixturewas refluxed under N₂ for 6 hrs or until no dimer was revealed by TLC.The reaction was then cooled and the solvent evaporated. The excesssalicylidene was removed by gentle heating under vacuum. The residualsolid was redissolved in CH₂Cl₂ and the insoluble inorganic materialswere removed by filtration. The filtrate was concentrated and columnchromatographed using CH₂Cl₂ as the eluent to yield the product asbright yellow solids (0.07 g, 85%). MS (EI): m/z (relative intensity)663 (M⁺, 75), 529 (100), 332 (35). The emission spectrum is in FIG. 18and the proton NMR spectrum is in FIG. 19.

thpyIrsd. The chloride bridge dimer (thpyIrCl)₂ (0.21 g, 0.19 mmol) wastreated the same way as (tpyIrCl)₂. Yield: 0.21 g, 84%. MS (EI): m/z(relative intensity) 647 (M⁺, 100), 513 (30), 486 (15), 434 (20), 324(25). The emission spectrum is in FIG. 20 and the proton NMR spectrum isin FIG. 21.

btIrsd. The chloride bridge dimer (btIrCl)₂ (0.05 g, 0.039 mmol) wastreated the same way as (tpyIrCl)₂. Yield: 0.05 g, 86%. MS (EI): m/z(relative intensity) 747 (M⁺, 100), 613 (100), 476 (30), 374 (25), 286(32). The emission spectrum is in FIG. 22 and the proton NMR spectrum isin FIG. 23.

Ir(bq)₂(acac), BQIr. The chloride bridged dimer (Ir(bq)₂Cl)₂ (0.091 g,0.078 mmol), acetylacetone (0.021 g) and sodium carbonate (0.083 g) weremixed in 10 ml of 2-ethoxyethanol. The mixture was refluxed under N₂ for10 hrs or until no dimer was revealed by TLC. The reaction was thencooled and the yellow precipitate filtered. The product was purified byflash chromatography using dichloromethane. Product: bright yellowsolids (yield 91%). ¹H NMR (360 MHz, acetone-d₆), ppm: 8.93 (d, 2H),8.47 (d, 2H), 7.78 (m, 4H), 7.25 (d, 2H), 7.15 (d, 2H), 6.87 (d, 2H),6.21 (d, 2H), 5.70 (s, 1H), 1.63 (s, 6H). MS, e/z: 648 (M⁺, 80%), 549(100%). The emission spectrum is in FIG. 24 and the proton NMR spectrumis in FIG. 25.

Ir(bq)₂(Facac), BQIrFA. The chloride bridged dimer (Ir(bq)₂Cl)₂ (0.091g, 0.078 mmol), hexafluoroacetylacetone (0.025 g) and sodium carbonate(0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixture wasrefluxed under N₂ for 10 hrs or until no dimer was revealed by TLC. Thereaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: yellow solids (yield 69%). ¹H NMR (360 MHz, acetone-d₆), ppm:8.99 (d, 2H), 8.55 (d, 2H), 7.86 (m, 4H), 7.30 (d, 2H), 7.14 (d, 2H),6.97 (d, 2H), 6.13 (d, 2H), 5.75 (s, 1H). MS, e/z: 684 (M+, 59%), 549(100%). Emission spectrum in FIG. 26.

Ir(thpy)₂(acac), THPIr. The chloride bridged dimer (Ir(thpy)₂Cl)₂ (0.082g, 0.078 mmol), acetylacetone (0.025 g) and sodium carbonate (0.083 g)were mixed in 10 ml of 2-ethoxyethanol. The mixture was refluxed underN₂ for 10 hrs or until no dimer was revealed by TLC. The reaction wasthen cooled and the yellow precipitate filtered. The product waspurified by flash chromatography using dichloromethane. Product:yellow-orange solid (yield 80%). ¹H NMR (360 MHz, acetone-d₆), ppm: 8.34(d, 2H), 7.79 (m, 2H), 7.58 (d, 2H), 7.21 (d, 2H), 7.15 (d, 2H), 6.07(d, 2H), 5.28 (s, 1H), 1.70 (s, 6H). MS, e/z: 612 (M+, 89%), 513 (100%).The emission spectrum is in FIG. 27 (noted “THIr”) and the proton NMRspectrum is in FIG. 28.

Ir(ppy)₂(acac), PPIr. The chloride bridged dimer (Ir(ppy)₂Cl)₂ (0.080 g,0.078 mmol), acetylacetone (0.025 g) and sodium carbonate (0.083 g) weremixed in 10 ml of 2-ethoxyethanol. The mixture was refluxed under N₂ for10 hrs or until no dimer was revealed by TLC. The reaction was thencooled and the yellow precipitate filtered. The product was purified byflash chromatography using dichloromethane. Product: yellow solid (yield87%). ¹H NMR (360 MHz, acetone-d₆), ppm: 8.54 (d, 2H), 8.06 (d, 2H),7.92 (m, 2H), 7.81 (d, 2H), 7.35 (d, 2H), 6.78 (m, 2H), 6.69 (m, 2H),6.20 (d, 2H), 5.12 (s, 1H), 1.62 (s, 6H). MS, e/z: 600 (M+, 75%), 501(100%). The emission spectrum is in FIG. 29 and the proton NMR spectrumis in FIG. 30.

Ir(bthpy)₂(acac), BTPIr. The chloride bridged dimer (Ir(bthpy)₂Cl)₂(0.103 g, 0.078 mmol), acetylacetone (0.025 g) and sodium carbonate(0.083 g) were mixed in 10 ml of 2-ethoxyethanol. The mixture wasrefluxed under N₂ for 10 hrs or until no dimer was revealed by TLC. Thereaction was then cooled and the yellow precipitate filtered. Theproduct was purified by flash chromatography using dichloromethane.Product: yellow solid (yield 49%). MS, e/z: 712 (M+, 66%), 613 (100%).Emission spectrum is in FIG. 31.

[Ir(ptpy)₂Cl]₂. A solution of IrCl₃.xH₂O (1.506 g, 5.030 mmol) and2-(p-tolyl)pyridine (3.509 g, 20.74 mmol) in 2-ethoxyethanol (30 mL) wasrefluxed for 25 hours. The yellow-green mixture was cooled to roomtemperature and 20 mL of 1.0 M HCl was added to precipitate the product.The mixture was filtered and washed with 100 mL of 1.0 M HCl followed by50 mL of methanol then dried. The product was obtained as a yellowpowder (1.850 g, 65%).

[Ir(ppz)₂Cl]₂. A solution of IrCl₃.xH₂O (0.904 g, 3.027 mmol) and1-phenylpyrazole (1.725 g, 11.96 mmol) in 2-ethoxyethanol (30 mL) wasrefluxed for 21 hours. The gray-green mixture was cooled to roomtemperature and 20 mL of 1.0 M HCl was added to precipitate the product.The mixture was filtered and washed with 100 mL of 1.0 M HCl followed by50 mL of methanol then dried. The product was obtained as a light graypowder (1.133 g, 73%).

[Ir(C6)₂Cl]₂. A solution of IrCl₃.xH₂O (0.075 g, 0.251 mmol) andcoumarin C6 [3-(2-benzothiazolyl)-7-(diethyl)coumarin] (Aldrich) (0.350g, 1.00 mmol) in 2-ethoxyethanol (15 mL) was refluxed for 22 hours. Thedark red mixture was cooled to room temperature and 20 mL of 1.0 M HClwas added to precipitate the product. The mixture was filtered andwashed with 100 mL of 1.0 M HCl followed by 50 mL of methanol. Theproduct was dissolved in and precipitated with methanol. The solid wasfiltered and washed with methanol until no green emission was observedin the filtrate. The product was obtained as an orange powder (0.0657 g,28%).

Ir(ptpy)₂(acac) (tpyIr). A solution of [Ir(ptpy)₂Cl]₂ (1.705 g, 1.511mmol), 2,4-pentanedione (3.013 g, 30.08 mmol) and (1.802 g, 17.04 mmol)in 1,2-dichloroethane (60 mL) was refluxed for 40 hours. Theyellow-green mixture was cooled to room temperature and the solvent wasremoved under reduced pressure. The product was taken up in 50 mL ofCH₂Cl₂ and filtered through Celite. The solvent was removed underreduced pressure to yield orange crystals of the product (1.696 g, 89%).The emission spectrum is given in FIG. 32. The results of an x-raydiffraction study of the structure are given in FIG. 33. One sees thatthe nitrogen atoms of the tpy (“tolyl pyridyl”) groups are in a transconfiguration. For the x-ray study, the number of reflections was 4663and the R factor was 5.4%.

Ir(C6)₂(acac) (C6Ir). Two drops of 2,4-pentanedione and an excess ofNa₂CO₃ were added to solution of [Ir(C6)₂Cl]₂ in CDCl₃. The tube washeated for 48 hours at 50° C. and then filtered through a short plug ofCelite in a Pasteur pipette. The solvent and excess 2,4-pentanedionewere removed under reduced pressure to yield the product as an orangesolid. Emission of C6 in FIG. 34 and of C6Ir in FIG. 35.

Ir(ppz)₂picolinate (PZIrp). A solution of [Ir(ppz)₂Cl]₂ (0.0545 g,0.0530 mmol) and picolinic acid (0.0525 g, 0.426 mmol) in CH₂Cl₂ (15 mL)was refluxed for 16 hours. The light green mixture was cooled to roomtemperature and the solvent was removed under reduced pressure. Theresultant solid was taken up in 10 mL of methanol and a light greensolid precipitated from the solution. The supernatant liquid wasdecanted off and the solid was dissolved in CH₂Cl₂ and filtered througha short plug of silica. The solvent was removed under reduced pressureto yield light green crystals of the product (0.0075 g, 12%). Emissionin FIG. 36.

2-(1-naphthyl)benzoxazole, (BZO-Naph). (11.06 g, 101 mmol) of2-aminophenol was mixed with (15.867 g, 92.2 mmol) of 1-naphthoic acidin the presence of polyphosphoric acid. The mixture was heated andstirred at 240° C. under N₂ for 8 hrs. The mixture was allowed to coolto 100° C., this was followed by addition of water. The insolubleresidue was collected by filtration, washed with water then reslurriedin an excess of 10% Na₂CO₃. The alkaline slurry was filtered and theproduct washed thoroughly with water and dried under vacuum. The productwas purified by vacuum distillation. BP 140° C./0.3 mm Hg. Yield 4.8 g(21%).

Tetrakis(2-(1-naphthyl)benzoxazoleC², N)(μ-dichloro)diiridium.((Ir₂(BZO-Naph)₄CI)₂). Iridium trichloride hydrate (0.388 g) wascombined with 2-(1-naphthyl)benzoxazole (1.2 g, 4.88 mmol). The mixturewas dissolved in 2-ethoxyethanol (30 mL) then refluxed for 24 hrs. Thesolution was cooled to room temperature, the resulting orange solidproduct was collected in a centrifuge tube. The dimer was washed withmethanol followed by chloroform through four cycles ofcentrifuge/redispersion cycles. Yield 0.66 g.

Bis(2-(1-naphthyl)benzoxazole) acetylacetonate, Ir(BZO-Naph)₂(acac),(BONIr). The chloride bridged dimer (Ir₂(BZO-Naph)₄Cl)₂ (0.66 g, 0.46mmol), acetylacetone (0.185 g) and sodium carbonate (0.2 g) were mixedin 20 ml of dichloroethane. The mixture was refluxed under N₂ for 60hrs. The reaction was then cooled and the orange/red precipitate wascollected in centrifuge tube. The product was washed with water/methanol(1:1) mixture followed by methanol wash through four cycles ofcentrifuge/redispersion cycles. The orange/red solid product waspurified by sublimation. SP 250° C./2×10⁻⁵ torr, yield 0.57 g (80%). Theemission spectrum is in FIG. 37 and the proton NMR spectrum is in FIG.38.

Bis(2-phenylbenzothiazole) Iridium acetylacetonate (BTIr). 9.8 mmol(0.98 g, 1.0 mL) of 2,4-pentanedione was added to a room-temperaturesolution of 2.1 mmol 2-phenylbenzothiazole Iridium chloride dimer (2.7g) in 120 mL of 2-ethoxyethanol. Approximately 1 g of sodium carbonatewas added, and the mixture was heated to reflux under nitrogen in an oilbath for several hours. Reaction mixture was cooled to room temperature,and the orange precipitate was filtered off via vacuum. The filtrate wasconcentrated and methanol was added to precipitate more product.Successive filtrations and precipitations afforded a 75% yield. Theemission spectrum is in FIG. 39 and the proton NMR spectrum is in FIG.40.

Bis(2-phenylbenzooxazole) Iridium acac (BOIr). 9.8 mmol (0.98 g, 1.0 mL)of 2,4-pentanedione was added to a room-temperature solution of 2.4 mmol2-phenylbenzoxazole Iridium chloride dimer (3.0 g) in 120 mL of2-ethoxyethanol. Approximately 1 g of sodium carbonate was added, andthe mixture was heated to reflux under nitrogen in an oil bath overnight(˜16 hrs.). Reaction mixture was cooled to room temperature, and theyellow precipitate was filtered off via vacuum. The filtrate wasconcentrated and methanol was added to precipitate more product.Successive filtrations and precipitations afforded a 60% yield. Theemission spectrum is in FIG. 41, and the proton NMR spectrum is in FIG.42.

Bis(2-phenylbenzothiazole) Iridium (8-hydroxyquinolate) (BTIrQ). 4.7mmol (0.68 g) of 8-hydroxyquinoline was added to a room-temperaturesolution of 0.14 mmol 2-phenylbenzothiazole Iridium chloride dimer (0.19g) in 20 mL of 2-ethoxyethanol. Approximately 700 mg of sodium carbonatewas added, and the mixture was heated to reflux under nitrogen in an oilbath overnight (23 hrs.). Reaction mixture was cooled to roomtemperature, and the red precipitate was filtered off via vacuum. Thefiltrate was concentrated and methanol was added to precipitate moreproduct. Successive filtrations and precipitations afforded a 57% yield.The emission spectrum is in FIG. 43 and the proton NMR spectrum is inFIG. 44.

Bis(2-phenylbenzothiazole) Iridium picolinate (BTIrP). 2.14 mmol (0.26g) of picolinic acid was added to a room-temperature solution of 0.80mmol 2-phenylbenzothiazole Iridium chloride dimer (1.0 g) in 60 mL ofdichloromethane. The mixture was heated to reflux under nitrogen in anoil bath for 8.5 hours. The reaction mixture was cooled to roomtemperature, and the yellow precipitate was filtered off via vacuum. Thefiltrate was concentrated and methanol was added to precipitate moreproduct. Successive filtrations and precipitations yielded about 900 mgof impure product. Emission spectrum is in FIG. 45.

Bis(2-phenylbenzooxazole) Iridium picolinate (BOIrP). 0.52 mmol (0.064g) of picolinic acid was added to a room-temperature solution of 0.14mmol 2-phenylbenzoxazole Iridium chloride dimer (0.18 g) in 20 mL ofdichloromethane. The mixture was heated to reflux under nitrogen in anoil bath overnight (17.5 hrs.). Reaction mixture was cooled to roomtemperature, and the yellow precipitate was filtered off via vacuum. Theprecipitate was dissolved in dichloromethane and transferred to a vial,and the solvent was removed. Emission spectrum is in FIG. 46.

Comparative emission spectra for different L′ in btIr complexes areshown in FIG. 47.

These syntheses just discussed have certain advantages over the priorart. Compounds of formula PtL₃ cannot be sublimed without decomposition.Obtaining compounds of formula IrL₃ can be problematic. Some ligandsreact cleanly with Ir(acac)₃ to give the tris complex, but more thanhalf of the ligands we have studied do not react cleanly in thereaction:3L+Ir(acac)₃→L₃Ir+(acac)H;typically 30% yield, L=2-phenylpyridine, benzoquinoline,2-thienylpyridine. A preferred route to Ir complexes can be through thechloride-bridged dimer L₂M(μ—Cl)₂ML₂ via the reaction:4L+IrCl₃ .nH₂O→L₂M(μ—Cl)₂ML₂+4HCl.

Although fewer than 10% of the ligands we have studied failed to givethe Ir dimer cleanly and in high yield, the conversion of the dimer intothe tris complex IrL₃ is problematic working for only a few ligands.L₂M(μ—Cl)₂ML₂+2Ag⁺+2L→L₃Ir+2AgCl.

We have discovered that a far more fruitful approach to preparingphosphorescent complexes is to use chloride bridged dimers to createemitters. The dimer itself does not emit strongly, presumably because ofstrong self quenching by the adjacent metal (e.g., iridium) atoms. Wehave found that the chloride ligands can be replaced by a chelatingligand to give a stable, octahedral metal complex through the chemistry:L₂M(μ—Cl)₂ML₂+XH→L₂MX+HCl

We have extensively studied the system wherein M=iridium. The resultantiridium complexes emit strongly, in most cases with lifetimes of 1-3microseconds (“μsec”). Such a lifetime is indicative of phosphorescence(see Charles Kittel, Introduction to Solid State Physics). Thetransition in these materials is a metal ligand charge transfer(“MLCT”).

In the discussion that follows below, we analyze data of emissionspectra and lifetimes of a number of different complexes, all of whichcan be characterized as L₂MX (M=Ir), where L is a cyclometallated(bidentate) ligand and X is a bidentate ligand. In nearly every case,the emission in these complexes is based on an MLCT transition betweenIr and the L ligand or a mixture of that transition and an intraligandtransition. Specific examples are described below. Based on theoreticaland spectroscopic studies, the complexes have an octahedral coordinationabout the metal (for example, for the nitrogen heterocycles of the Lligand, there is a trans disposition in the Ir octahedron).Specifically, in FIG. 11, we give the structure for L₂IrX, whereinL=2-phenyl pyridine and X=acac, picolinate (from picolinic acid),salicylanilide, or 8-hydroxyquinolinate.

A slight variation of the synthetic route to make L₂IrX allows formationof meridianal isomers of formula L₃Ir. The L₃Ir complexes that have beendisclosed previously all have a facial disposition of the chelatingligands. Herewith, we disclose the formation and use of meridianal L₃Ircomplexes as phosphors in OLEDs. The two structures are shown in FIG.12.

The facial L₃Ir isomers have been prepared by the reaction of L withIr(acac)₃ in refluxing glycerol as described in equation 2 (below). Apreferred route into L₃Ir complexes is through the chloride bridgeddimer (L₂Ir(m-Cl)₂IrL₂), equation 3+4 (below). The product of equation 4is a facial isomer, identical to the one formed from Ir(acac)₃. Thebenefit of the latter prep is a better yield of facial-L₃Ir. If thethird ligand is added to the dimer in the presence of base andacetylacetone (no Ag⁺), a good yield of the meridianal isomer isobtained. The meridianal isomer does not convert to the facial one onrecrystallization, refluxing in coordinating solvents or on sublimation.Two examples of these meridianal complexes have been formed, mer-Irppyand mer-Irbq (FIG. 13); however, we believe that any ligand that gives astable facial-L₃Ir can be made into a meridianal form as well.3L+Ir(acac)₃→facial-L₃Ir+acacH  (2)

typically 30% yield, L=2-phenylpyridine, bezoquinoline,2-thienylpyridine4L+IrCl₃ .nH₂O→L₂Ir(m-Cl)₂IrL₂+4HCl  (3)

typically>90% yield, see attached spectra for examples of L, also workswell for all ligands that work in equation (2)L₂Ir(m-Cl)₂IrL₂+2Ag⁺+2L→2 facial-L₃Ir+2AgCl  (4)

typically 30% yield, only works well for the same ligands that work wellfor equation (2)L₂Ir(m-Cl)₂IrL₂+XH+Na₂CO₃+L→meridianal-L₃Ir  (5)

typically>80% yield, XH=acetylacetone

Surprisingly, the photophysics of the meridianal isomers is differentfrom that of the facial forms. This can be seen in the details of thespectra discussed below, which show a marked red shift and broadening inthe meridianal isomer relative to its facial counterpart. The emissionlines appear as if a red band has been added to the band characteristicof the facial-L₃Ir. The structure of the meridianal isomer is similar tothose of L₂IrX complexes, with respect, for example, to the arrangementof the N atoms of the ligands about Ir. Specifically, for L=ppy ligands,the nitrogen of the L ligand is trans in both mer-Ir(ppy)₃ and in(Ppy)₂Ir(acac). Further, one of the L ligands for the mer-L₃Ir complexeshas the same coordination as the X ligand of L₂IrX complexes. In orderto illustrate this point a model of mer-Ir(ppy)₃ is shown next to(ppy)₂Ir(acac) in FIG. 14. One of the ppy ligands of mer-Ir(ppy)₃ iscoordinated to the Ir center in the same geometry as the acac ligand of(ppy)₂Ir(acac).

The HOMO and LUMO energies of these L₃Ir molecules are clearly affectedby the choice of isomer. These energies are very important iscontrolling the current-voltage characteristics and lifetimes of OLEDsprepared with these phosphors. The syntheses for the two isomersdepicted in FIG. 13 are as follows.

Syntheses of Meridianal Isomers

mer-Irbq: 91 mg (0.078 mmol) of [Ir(bq)₂Cl]₂ dimer, 35.8 mg (0.2 mmol)of 7,8-benzoquinoline, 0.02 ml of acetylacetone (ca. 0.2 mmol) and 83 mg(0.78 mmol) of sodium carbonate were boiled in 12 ml of 2-ethoxyethanol(used as received) for 14 hours in inert atmosphere. Upon coolingyellow-orange precipitate forms and is isolated by filtration and flashchromatography (silica gel, CH₂Cl₂) (yield 72%). 1H NMR (360 MHz,dichloromethane-d2), ppm: 8.31 (q, 1H), 8.18 (q, 1H), 8.12 (q, 1H), 8.03(m, 2H), 7.82 (m, 3H), 7.59 (m, 2H), 7.47 (m, 2H), 7.40 (d, 1H), 7.17(m, 9H), 6.81 (d, 1H), 6.57 (d, 1H). MS, e/z: 727 (100%, M+). NMRspectrum in FIG. 48.

mer-Ir(tpy)₃: A solution of IrCl₃.xH₂O (0.301 g, 1.01 mmol),2-(p-tolyl)pyridine (1.027 g, 6.069 mmol), 2,4-pentanedione (0.208 g,2.08 mmol) and Na₂CO₃ (0.350 g, 3.30 mmol) in 2-ethoxyethanol (30 mL)was refluxed for 65 hours. The yellow-green mixture was cooled to roomtemperature and 20, mL of 1.0 M HCl was added to precipitate theproduct. The mixture was filtered and washed with 100 mL of 1.0 M HClfollowed by 50 mL of methanol then dried and the solid was dissolved inCH₂Cl₂ and filtered through a short plug of silica. The solvent wasremoved under reduced pressure to yield the product as a yellow-orangepowder (0.265 g, 38%).

This invention is further directed toward the use of the above-noteddopants in a host phase. This host phase may be comprised of moleculescomprising a carbazole moiety. Molecules which fall within the scope ofthe invention are included in the following.

[A line segment denotes possible substitution at any available carbonatom or atoms of the indicated ring by alkyl or aryl groups.]

An additional preferred molecule with a carbazole functionality is4,4′-N,N′-dicarbazole-biphenyl (CBP), which has the formula:

The light emitting device structure that we chose to use is very similarto the standard vacuum deposited one. As an overview, a holetransporting layer (“HTL”) is first deposited onto the ITO (indium tinoxide) coated glass substrate. For the device yielding 12% quantumefficiency, the HTL consisted of 30 nm (300 Å) of NPD. Onto the NPD athin film of the organometallic compound doped into a host matrix isdeposited to form an emitter layer. In the example, the emitter layerwas CBP with 12% by weight bis(2-phenylbenzothiazole) iridiumacetylacetonate (termed “BTIr”), and the layer thickness was 30 nm (300Å). A blocking layer is deposited onto the emitter layer. The blockinglayer consisted of bathcuproine (“BCP”), and the thickness was 20 nm(200 Å). An electron transport layer is deposited onto the blockinglayer. The electron transport layer consisted of Alq₃ of thickness 20nm. The device is finished by depositing a Mg—Ag electrode onto theelectron transporting layer. This was of thickness 100 nm. All of thedepositions were carried out at a vacuum less than 5×10⁻⁵ Torr. Thedevices were tested in air, without packaging.

When we apply a voltage between the cathode and the anode, holes areinjected from ITO to NPD and transported by the NPD layer, whileelectrons are injected from MgAg to Alq and transported through Alq andBCP. Then holes and electrons are injected into EML and carrierrecombination occurs in CBP, the excited states were formed, energytransfer to BTIr occurs, and finally BTIr molecules are excited anddecay radiatively.

As illustrated in FIG. 15, the quantum efficiency of this device is 12%at a current density of about 0.01 mA/cm². Pertinent terms are asfollows: ITO is a transparent conducting phase of indium tin oxide whichfunctions as an anode; ITO is a degenerate semiconductor formed bydoping a wide band semiconductor; the carrier concentration of the ITOis in excess of 10¹⁹/cm³; BCP is an exciton blocking and electrontransport layer; Alq₃ is an electron injection layer; other holetransport layer materials could be used, for example, TPD, a holetransport layer, can be used.

BCP functions as an electron transport layer and as an exciton blockinglayer, which layer has a thickness of about 10 nm (100 Å). BCP is2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also calledbathocuproine) which has the formula:

The Alq₃, which functions as an electron injection/electron transportlayer has the following formula:

In general, the doping level is varied to establish the optimum dopinglevel.

As noted above, fluorescent materials have certain advantages asemitters in devices. If the L ligand that is used in making the L₂MX(for example, M=Ir) complex has a high fluorescent quantum efficiency,it is possible to use the strong spin orbit coupling of the Ir metal toefficiently intersystem cross in and out of the triplet states of theligands. The concept is that the Ir makes the L ligand an efficientphosphorescent center. Using this approach, it is possible to take anyfluorescent dye and make an efficient phosphorescent molecule from it(that is, L fluorescent but L₂MX (M=Ir) phosphorescent).

As an example, we prepared a L₂IrX wherein L=coumarin and X=acac. Werefer to this as coumarin-6 [“C6Ir”]. The complex gives intense orangeemission, whereas coumarin by itself emits green. Both coumarin and C6Irspectra are given in the Figures.

Other fluorescent dyes would be expected to show similar spectralshifts. Since the number of fluorescent dyes that have been developedfor dye lasers and other applications is quite large, we expect thatthis approach would lead to a wide range of phosphorescent materials.

One needs a fluorescent dye with suitable functionality such that it canbe metallated by the metal (for example, iridium) to make a 5- or6-membered metallocycle. All of the L ligands we have studied to datehave sp² hybridized carbons and heterocyclic N atoms in the ligands,such that one can form a five membered ring on reacting with Ir.

Potential degradation reactions, involving holes or electrons, can occurin the emitter layer. The resultant oxidation or reduction can alter theemitter, and degrade performance. In order to get the maximum efficiencyfor phosphor doped OLEDs, it is important to control the holes orelectrons which lead to undesirable oxidation or reduction reactions.One way to do this is to trap carriers (holes or electrons) at thephosphorescent dopant. It may be beneficial to trap the carrier at aposition remote from the atoms or ligands responsible for thephosphorescence. The carrier that is thus remotely trapped could readilyrecombine with the opposite carrier either intramolecularly or with thecarrier from an adjacent molecule.

An example of a phosphor designed to trap holes is shown in FIG. 16. Thediarylamine group on the salicylanlide group is expected to have a HOMOlevel 200 to 300 mV above that of the Ir complex (based onelectrochemical measurements), leading to the holes being trappedexclusively at the amine groups. Holes will be readily trapped at theamine, but the emission from this molecule will come from MLCT andintraligand transitions from the Ir(phenylpyridine) system. An electrontrapped on this molecule will most likely be in one of the pyridylligands. Intramolecular recombination will lead to the formation of anexciton, largely in the Ir(phenylpyridine) system. Since the trappingsite is on the X ligand, which is typically not involved extensively inthe luminescent process, the presence of the trapping site will notgreatly affect the emission energy for the complex. Related moleculescan be designed in which electron carriers are trapped remoted to theL₂Ir system.

As found in the IrL₃ system, the emission color is strongly affected bythe L ligand. This is consistent with the emission involving either MLCTor intraligand transitions. In all of the cases that we have been ableto make both the tris complex (i.e., IrL₃) and the L₂IrX complex, theemission spectra are very similar. For example Ir(ppy)₃ and(ppy)₂Ir(acac) (acronym=PPIr) give strong green emission with a λ_(max)of 510 nm. A similar trend is seen in comparing Ir(BQ)₃ and Ir(thpy)₃ totheir L₂Ir(acac) derivatives, i.e., in some cases, no significant shiftin emission between the two complexes.

However, in other cases, the choice of X ligand affects both the energyof emission and efficiency. Acac and salicylanilide L₂IrX complexes givevery similar spectra. The picolinic acid derivatives that we haveprepared thus far show a small blue shift (15 nm) in their emissionspectra relative to the acac and salicylanilide complexes of the sameligands. This can be seen in the spectra for BTIr, BTIrsd and BTIrpic.In all three of these complexes we expect that the emission becomesprincipally form MLCT and Intra-L transitions and the picolinic acidligands are changing the energies of the metal orbitals and thusaffecting the MLCT bands.

If an X ligand is used whose triplet levels fall lower in energy thanthe “L₂Ir” framework, emission from the X ligand can be observed. Thisis the case for the BTIRQ complex. In this complex the emissionintensity is very weak and centered at 650 nm. This was surprising sincethe emission for the BT ligand based systems are all near 550 nm. Theemission in this case is almost completely from Q based transitions. Thephosphorescence spectra for heavy metal quinolates (e.g., IrQ₃ or PtQ₂)are centered at 650 nm. The complexes themselves emit with very lowefficiency, <0.01. Both the energy and efficiency of the L₂IrQ materialis consistent “X” based emission. If the emission from the X ligand orthe “IrX” system were efficient this could have been a good red emitter.It is important to note that while all of the examples listed here arestrong “L” emitters, this does not preclude a good phosphor from beingformed from “X” based emission.

The wrong choice of X ligand can also severely quench the emission fromL₂IrX complexes. Both hexafluoro-acac and diphenyl-acac complexes giveeither very weak emission of no emission at all when used as the Xligand in L₂IrX complexes. The reasons why these ligands quench emissionso strong are not at all clear, one of these ligands is more electronwithdrawing than acac and the other more electron donating. We give thespectrum for BQIrFA in the Figures. The emission spectrum for thiscomplex is slightly shifted from BQIr, as expected for the much strongerelectron withdrawing nature of the hexafluoro-acac ligand. The emissionintensity from BQIrFA is at least 2 orders of magnitude weaker thanBQIr. We have not explored the complexes of these ligands due to thissevere quenching problem.

CBP was used in the device described herein. The invention will workwith other hole-transporting molecules known by one of ordinary skill towork in hole transporting layers of OLEDs. Specifically, the inventionwill work with other molecules comprising a carbazole functionality, oran analogous aryl amine functionality.

The OLED of the present invention may be used in substantially any typeof device which is comprised of an OLED, for example, in OLEDs that areincorporated into a larger display, a vehicle, a computer, a television,a printer, a large area wall, theater or stadium screen, a billboard ora sign.

1. An organic light emitting device comprising an anode, a cathode, andan emissive layer, wherein the emissive layer is located between theanode and the cathode, and the emissive layer comprises a phosphorescentorganometallic iridium compound, wherein the phosphorescentorganometallic iridium compound has an aromatic ligand.
 2. The organiclight emitting device of claim 1, wherein the device further includes atleast one electron transporting layer and at least one hole transportinglayer between the anode and cathode.
 3. The organic light emittingdevice of claim 1 wherein the phosphorescent organometallic iridiumcompound is a phosphorescent cyclometallated iridium compound.
 4. Theorganic light emitting device of claim 2 wherein the phosphorescentorganometallic iridium compound is a phosphorescent cyclometallatediridium compound.
 5. The organic light emitting device of claim 3,wherein the phosphorescent cyclometallated iridium compound includes acycle closed by at least one metal-nitrogen bond.
 6. The organic lightemitting device of claim 4, wherein the phosphorescent cyclometallatediridium compound includes a cycle closed by at least one metal-nitrogenbond.
 7. The organic light emitting device of claim 1, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 8. The organic light emitting device of claim 2, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 9. The organic light emitting device of claim 3, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 10. The organic light emitting device of claim 4, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 11. The organic light emitting device of claim 5, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 12. The organic light emitting device of claim 6, wherein theemissive layer further comprises a host material, and the phosphorescentorganometallic iridium compound is present as a dopant in the hostmaterial.
 13. The organic light emitting device of any one of claim 2,wherein the phosphorescent cyclometallated organometallic iridiumcompound is denoted by the formula:

wherein R is an alkyl or aryl group.
 14. The organic light emittingdevice of claim 13, wherein the R group is an alkyl group.
 15. Theorganic light emitting device of claim 13, wherein the R group is anaryl group.
 16. The organic light emitting device of any one of claim 8,wherein the phosphorescent cyclometallated organometallic iridiumcompound is denoted by the formula:

wherein R is an alkyl or aryl group.
 17. The organic light emittingdevice of claim 16, wherein the R group is an alkyl group.
 18. Theorganic light emitting device of claim 16, wherein the R group is anaryl group.